1. Field of the Invention
The present invention relates to compositions for soda-lime-silica glass suitable for the production of glazings which can be used in motor vehicles. These glass compositions contain between about 0.3 and 1% by weight total iron expressed in the form Fe.sub.2 O.sub.3, at least 35% of said iron being in the form FeO, and about 0.5 to 2.8 wt % fluorine. This glass composition is used to make a glazing having at least one glass sheet with a total light transmission factor under illuminant A of at least 70% for a thickness between 0.8 and 6 mm.
2. Discussion of the Background
Glazings used for automotive applications must satisfy legal requirements with respect to their light transmission. Thus, a glazing used as a windshield must, in certain countries, have a total light transmission factor under illuminant A (TL.sub.A as defined by the International Commission on Illumination) of at least 75%. The glazings used for the production of side and rear windows must, under the same conditions, have a factor TL.sub.A of at least 70%.
As the amount of glass used in motor vehicles is at present very large and customers are making ever greater demands regarding comfort, all vehicle designers are seeking ways of making it possible to reduce the amount of heat felt by passengers. One of these ways is to use glazings with a total energy transmission factor (T.sub.E) which is as low as possible.
In order to maintain a high light transmission in the visible part of the spectrum, while absorbing to the greatest possible extent the remainder of the solar energy, it is known to introduce iron into a glass composition. The iron is present in the glass both as ferric oxide (Fe.sub.2 O.sub.3) and ferrous oxide (FeO).
The presence of Fe.sub.2 O.sub.3 makes it possible to absorb UV radiation and radiation having short wavelengths in the visible part of the spectrum. The presence of FeO makes it possible to absorb near IR radiation and radiation corresponding to the long wavelengths of the visible range. Although an increase in the content of iron in these two oxide forms improves the absorption of radiation at the two ends of the visible spectrum, this effect is obtained to the detriment of the light transmission.
Hitherto a number of solutions have been proposed for the optimum utilization of the capacity of iron oxides to absorb radiation, while still maintaining the greatest possible light transmission.
Solutions favoring the absorption of radiation in the near IR range are described in U.S. Pat. Nos. 3,652,303 and 4,792,536. The glasses described in these patents are produced under conditions such that the FeO proportion compared with the total iron is very high, being at least equal to 80% in the first patent and at least 35% in the second patent.
The iron content of the glasses described in U.S. Pat. No. 3,652,303 can vary widely (0.1 to 2% by weight). The transmission curves in this patent show that the "blue glass" according to the invention has a higher adsorption in the IR range than the two glasses given as references. It should be noted that the absorption band of the blue glass extends much more into the visible range than that of the glass called "green tinted plate".
In U.S. Pat. No. 4,792,536, the iron content varies within much narrower limits (0.45 to 0.65%). It can also be observed that the absorption band of glass No. 5, which illustrates this invention, extends further into the visible range than that of glass No. 3 illustrating the prior art.
A need continues to exist for glass compositions used to make glazings having a high total transmission factor while absorbing substantial IR radiation.